Pulsing electronic measuring and control apparatus



June 16, 1953 e. A. F. MACHLET 2,642,228

Punsmc mc'momc MEASURING AND CONTROL Arpmws Filed Dec. so, 1947 i 2 Shots-Sheet 1 Patented June 1c, 1953 2,642,228

PULSINGELECTRONIC MEASURING AND CONTROL APPARATUS V George A. Fr-Machlet, Elizabeth, N. J. Application December 30, 1947, Serial No. 794,462

1-8 claims. (01. 230-68) This invention relatesto electronic measuring and control apparatus, and more particularly to 7 electronic apparatus of high sensitivity whichresponds to variations in a control factor, such as temperature, pressure, direction, current, voltage or the like, to register or record the changes in the magnitude of the factor and/or to energize a control system in accordance with the change in the control factor.

2. resistance heater type and much fuel-fired equipment may also be advantageously controlled by a modulated current pulse system. The modulated currentpulse system of control is also of advantage when the controlled condition or processis subject to considerable inertia which would require, in the prior control systems, complex and/or expensive arrangements to eliminate hunting.

Electronic apparatus of this general type is he Objects of the present invention are to provide quently employed to regulate the supply .of fuel electronic measuring and proportioning control to a burner to maintain a desired temperature apparatus in which the measuring or control dewithin an oven or furnace, and some of the prior vice is energized by current pulses. Objects are equipment has provided a proportioning conto provide electronic control apparatus which trol, i. e. a modulation of the fuel supply is proafford a proportioning control but which do not portion'to the departure of the oven or furnace require a rebalancing circuit connection from temperature from the preselected control value. the controlled device to the electronic control Some f the prior proportionting control systems circuits. Other and more specific objects of the included an electronic circuit whi h wa unbalinvention are to provide electronic control sysanced to a greater or less extent according to the toms in which a vnormally,loalanced input circuit departure :of the oven or furnace temperature network for anelectronic tube is unbalanced by from its preselected value, a reversing motor ena departure of a control factor from a preselected ergized by an unbalance of the electronic circuit value, :current pulses which vary in duration or to actuate .a modulatingwfuel valve, and a return freq y With the eXteht 0f the unbalance are or rebalancing circuit including an impedance transmitted to a controlled device, and the input variable with the adjustment of the fuel valve ir uit IGtW Tk is ebala ced by an impedance and serving to rebalance the electronic circuit w c v es in ma tude wit t e t me-inteupon adjustment of the fuel valve by an incregrated average value of the current pulses. ment proportional to the temperature change These and other objects and advantages of the which produced the unbalance of the electronic invention Will b ppa from the following circuit. Electronic measuring and control .cir-. p ifi i Wh n k n w h he mp yi cuits of this type are described and claimed in drawings in which: my prior Patent No. 2,414,314, now reissued under Fi 1 i a ircui d a ram of an embodime t No. 23,160, and .in my copending application ser; of the invention in which the control factor 0p -v No. 499,557, filed August 21, 1943. which maerates through a photoelectric system to control tured as Patent No. 2,434,941 on January .27, the electrical heating of a crucible;

1948. Fig. 2 is a circuit diagram of another embodi- The present invention relates to proportioning ment in which the modulated current pulses are control circuits in which the controlled device, integrated to determine the adjustment of a con-' which may be a fuel valve or other regulating trol element; apparatus according to the particular application Fig. 3 is a circuit diagram of a further embodiof the invention, is not progressively adjusted but ment which, in association with a prior proporis shifted from one end position to the other tioning control system, provides a proportioning by modulated current pulses which vary as to control of energizationof the ultimate control duration'and/or frequency with the magnitude ,5 devi y curren p and of the departure of the measured control factor 418 a circuit agram of a further embodifrom its desired value. One embodiment of- .a' m t of theihvehtioh as i rp a d in a o control system of the modulated current pulse heating furnace assembly. typeis illustrated in Fig. 9 of my copending -ap-. In the control apparatus of Fig. 1, an electronic plication, and is specially claimed therein. i tube l is energized in conventional manner from V A dominantadvantage of the modulated cur an alternating current source S upon the closing rent pulse utypelof proportioning control :is :the of a switch 2 in a lead to the primary winding 3 elimination of the rebalancing circuit previously of a power transformer having a plurality of connected from the controlled device back to secondary windings 4,.5-and 6 for developing, rethe electronic circuit. Electric furnaces 10f the p c vfi y,

agrid bias voltage, a cathode heater voltage, and a plate circuit voltage. This tube I is shown as a triode having a control grid G, a plate or anode A, and a uniform potential cathode K with a heater H, but other types of amplifier tubes may be used. The controlled apparatus is a crucible I which is heated by a resistance winding 8 when switch 9 is closed by solenoid II! to connect the winding to a source S which may be the same as or, different from the current source S which energizes the tube I.

The input circuit network of the tube I is a Wheatstone bridge having photocells II, I2 as two adjacent arms and resistances l3, I4 as the other two adjacent arms. The secondarywinding d of the power transformer is connected across the bridge by leads I5 which extend respectively to the junctions of photocell I I and resistance I3, and of photocell I2 and resistance M. The bridge terminal a. common to the photocells I 5, I2 is connected to the control grid G by a lead It, and the opposite bridge terminal b is connected by lead I? to that terminal of the winding 5 which is at a negative potential during those half-cycles when the plate potential is positive. A resistance I8 extends between the bridge terminals a and b tobias the grid G negatively, by the voltage drop across the cathode heater H, during periods when the bridge network is balanced. This initial bias on grid G is such that the resulting or normal plate current is slightly'less than the pull-in current value of relay I9 and is slightly greater than the drop-out current value, and it may be approximately onehalf the maximum plate current for the applied plate voltage. point of the relay I9 is located approximately at the center of the grid voltage-plate current characteristic of the tube l..

The plate circuit of tube I includes a relay I9 having two sets of normally open contacts 20, 2!; the contacts 283 being connected inseries with the current source S and the operating coil II) of switch '9 by leads 22,22- The movableelement of relay contacts 2| is: connected by lead 23 to the ground side of the cathode heater 45 secondary 5, and the cooperating movable blade contact is connected through lead 23, heater re.- sistors 24, 25 and 26 in parallel, and lead H to the other side of the secondary winding. Resistors 24, 25 are in heat transfer relation to the bridge resistances I3, I l respectively, and the latter have positive temperature-resistance characteristics. The plate current increases upon an unbalance of the bridge which carries the grid G less negative, and the relay I9 pulls in at a preselected grid bias, thereby energizing solenoid III to close the power switch 9 to the crucible heater winding 8, and simultaneously supplying. cur rent to heaters 24, 26 over relay contacts 2|. The heater resistor 25 is the winding on a bimetallic strip 21 of a thermal switch having resilient blade contacts 28a,.28bin series with the heater 25 of the bridge arm It. The free end of the bimetallic strip has fixed theretoatip 29 of in,- sulating material which extends between the .ends of the switch blades 28a, 28b and floats freely between the switch blades when pulses of heating current are supplied to the heater 26 during the normal operation of the control apparatus. In each of its end positions, i. e. when at room temperature or when heated to saturation, the tip 29 of the bimetallic strip engages one of the blades to open the switch, thereby interrupting current to the heater 25. The heat capacity of the thermal switch is so great that into a crucible.

In other words, the operating the bimetallic strip 21, when heated, can open the contacts 28a, 2% only when heated continuously for a long period, for example during the heating up of a furnace, or after the introduction of a substantial mass of cold material The reason for rendering the heater 25 inoperative under such conditions will be apparent from the following description.

The resistances I3 and M are the rebalancing arm and the resetting arm, respectively, of the bridge network of the control system, and the efiective resistance values of these arms vary differentially to regulate the average power input to resistance winding 8 to maintain the temperature of the crucible l at a desired level. The rebalancing resistance I3 and its heater 24 have a lower heat inertia than the resetting resistance I 5 and its heater 25, and the resistance I3 therefore varies more rapidly as to temperature and resistance than does the resistance I l when timemodulated current pulses are supplied to, their respective heaters by the closing and opening of the relay contacts 2 I. At saturation, i. e. when the heaters 24, 25 receive current for an extended period, the bridge resistances I3, I4 are heated to the same maximum temperature and therefore have the same maximum resistanc when, as is convenient and preferable, the resistances I3, I4 are of the same magnitude at any given temperature. Each bridge resistance I3, I4 may be of nickel wire on a ceramic tube, and the heaters 24,, 25 may be wound on smaller diameter ceramic tubes which are then secured within the tubes of resistances I3, I,4 respectively.' Theheat inertia of each assembly may be adjusted to a desired value by fitting an iron core within the inner ceramic tube. The resistance assemblies should be so mounted in the control apparatus as to have equal lossesthrough radiation.

Inspection of the bridge network shows that current supplied'to. the bridge by the secondary winding 4 has no effect upon the bias voltage impressed upon grid G of tube Iso long as the bridge is balanced, .i. e. so long as no current from winding 4 flows along resistance I8 to establish a potential drop in the grid-cathode circuit comprising lead I6, resistance .I8 and lead I].

Furthermore, it is apparent that increases (or decreases) in the efiective magnitudes of the resistances I3 and I4 tend to unbalance the bridge network in opposite sense. Assuming instantaneous polarities for windings 4, 5 and 6 as in dicated by plus and minus symbols applied thereto, and remembering that the tube I can conduct only when the plate potential is positive, the bridge network will be unbalanced to make grid terminal a more positive if the resistance of the arm comprising photocell I2 is reduced below the resistance of the adjacent arm comprising photocell II. The bridge may be rebalanced by increasing the efiective resistance of the opposite bridge arm, comprising resistance I 3, and the degree'of unbalance may be increased by raising the resistance of the bridge arm I4.

The thermal inertias introduced by the heating of the rebalancing and resetting arms of the bridge to alte'r their effective values will have absolute and relative magnitudeswhich depend upon the particular process or apparatus to be controlled. The time constant of the resetting element must be equal to orgreaterthan the sum total of all time lags of the entire system if stability is to be obtained. The time constant of the-rebalancing element will be substantially smaller as the rebalanc upona change in operating conditions must be effected before the net work can be reset for an ultimate balance at the preselected control factor value. The requirements for rebalancing .and resetting a control system are discussed in my prior Patent No. 2,414,314.

During normal operation, the initialbalance condition of the bridge is determined by the selected control factor as-it varies above and below a desired value. As illustrated in Fig. 1, this primary control is provided by a temperaturemeasuring system which selectively illuminates the photocells II, I2 according to the crucible temperature. A thermocouple T within .the crucible I is connected to the moving coil-f a sensitive measuring instrument, and a inirrorM on the coil reflects a light beam from a light bulb L towards the mirrors 30, 31 from which the beam is reflected to photocells II, 12 respectively. The relative illumination of the photocell's depends upon the angular deflection of coil (7, and the measuring and light systems are so adjusted by any convenient means, not shown, that both photocells are illuminated to the same degree when the crucible l is at the preselected temperature.

The bridge network is balanced at this condi- 'tion if the bridge arms I3, I 4 are at the same temperature, and the tube is biased only by the potential drop across the cathode heater H. The plate circuit relay I9 is therefore de-energized'and its contacts '28, 2| are'open to interrupt current to the operating coil I-Il of the power switch 9 and to heaters 24, '25 of the control network. The control system is fail-safe in that power will not be supplied to the crucible heating winding 8 in the event of a failure of the light source,.-of-

the tube, or of other elements of the control system.

The measured temperature is far below thedesired control value when the crucible is cold, and the closing of switch 2 to place the apparatus in operation is immediately followed by an unbalance of the bridge network as the light beam is reflected only to the photocell I2 to reduce its resistance. The potential at terminal a of the bridge assumes a positive-value, or at least a less negative value, by the unbalance of the bridge, and the plate current of tube I rises to a high value corresponding to the new grid bias value; The relay I9 pulls in to energize the operating coil In of the power switch 9, andalso to supply current to the heater '24 of the bridge network and to the heater 26 of the thermal switch. The crucible heats up very slowly and will be far below the desired temperature when the rebalanc ing bridge arm I3 is heated to its maximum temperature by the heater 2E. The increased resistance of the bridge arm I3 is too small to rebalance the bridge in view of theiexcessive unbal'-' ance due to the marked differential between the illumination of photocells II and I2 when the measured temperature is far below the desired value. The resetting bridge arm I4 is inoperative, for all practical purposes, during the heating up of the crucible since the circuit of its heater 25 is initially open at the thermal switch contacts 28a, 28b, and the transitory closing of the contacts 28a, 28b before the thermal switch is heated to saturation merely increases the unbalance of the bridge network at'a time when it is already unbalanced by the differential illumination of photocells I I, I2 at the relatively low temperature of the crucible. The contacts 28a, 28b are again opened by the bimetallic strip 21 when heated to saturation, and the rebalancing, arm cools down to room'temperature before the de sired crucible temperature reached. It is of course possible, with more elaborate thermal switches than that schematically shown in Fig. 1 to avoid this transitory heating of the resetting bridge arm 14 during the heating up operation.

The crucible temperature eventually approachesthe desired value, and the instrument coii C moves clockwise from it previous end position to shift a portion of the light beam from the photocell 1'2 to the photocell II. The bridge network-is balanced when the measured temperature reaches the lower limit of the control rangai. e. before the photocells are equally illuminated, since the bridge arm I4 is cool but the rebalancing resistance arm I3 is heated to its maximum value. The plate current of tube I drops below the initial control point value at thev rebalance of the bridge, and the plate circuit re-l lay I9 drop out thereby to de-energize the operating coil ID of power switch 9 and to open the circuit to heater 24 of bridge arm I3 and to heater 25 of the thermal switch. The crucible temperature continues to rise for 501116 time after power is removed from the heating winding 8, since themass of the crucible introduces a considerable time lag,-but the heater 24 and the heater 26 of the thermal switch begin to cool down at once. The heat inertias or time constants of bridge arm I3 and thermal switch are so related tothe time lag of the crucible that the bridge arm I3 is cooled to room temperature and thermal contacts 28a,

28b reclose during the period within which the crucible temperature rises to somewhat above and then falls to the preselected control value. The

bridge network is again unbalanced when the crucible temperature drops below the selected control value, since the resistive bridge arms i3,

I4 are thenof equal value but the photocell arm I2 has a lesser resistance than the photocell arm 'II due to unequal distribution of the reflected light beam upon the photocells.

The further operation of the control system takes place in the following manner to provide a proportioning control of the resetting type. Tube I is rendered conductive by the unbalance of the bridge network, and relay I9 pulls in to close the circuits of operating coil ID of the power switch 9, and alsothe circuits of heaters 24, 25 of the bridge arms and of heater 26 of the thermal switch. The thermal switch no longer afiects the proportioning control since its thermal capacity is so great that it requires substantially continuous heating for a long period to open the circuit of heater 25 of the resetting system. The heat input to the crucible continues until the bridge network is 'rebalanced by the increased resistance of bridge arm I 3 due to heat transfer from the heat- V heating winding 8 and to the rebalancing heater 24 are interrupted by the opening of contacts 2B, 2| of relay I9. The heat inertia of the heater 24- resistance I3 combination is less than that of the crucible, .and the rebalance is therefore effected before the crucible temperature rises to the preselected control value. Heater 24 and resistance I3 cool down at arelatively rapid rate 'until'the resultant decreasein the effective value of resistance l3 again unbalances the bridge network, and the described cycle of operations is repeated.

The power input to the crucible is governed by the same relay l9 which regulates the current input to the heaters 24, 25 of the control network, and the power input pulses and control current pulses are therefore identically modulated. The extent and the sense of the unbalance of the bridge network by a differential illumination of the photocells ll, 12 depends directly upon the magnitude and sense of the departure of the measured temperature from the desired control value, and the time interval for the heating or cooling of the bridge arm l3 to rebalance the network therefore varies with the departure of the crucible temperature from the preselected control value. The regulated power input by current pulses, as averaged over a. cycle or number of cycles, is thus proportional to the average power input demand.

If the resetting heater 25 is omitted, as above assumed, the rebalance of the powerinput to a changed power demand may take place at any temperature within the control range which includes, and extends above and below, the preselected control temperature. The function of the resetting heater 25 is to insure a balance of the power input and power demand at the preselected control temperature. An increase in the effective value of the bridge resistance It operates in the same sense as a decrease in the resistance of the photocell i2 to unbalance the bridge network to increase the plate current of the tube l, i. e. to supply power to the crucible heating winding 8. The heat inertia or thermal lag of heater 25 and bridge arm hi is greater than the heat inertia or thermal lag of heater 24 and bridge arm [3, and the above-described rebalance of the bridge network by the heating of bridge arm l3 takes place in spite of the simultaneous heating, but at a slower rate, of bridge arm M. The ultimate resistance of bridge arm 14 after a plurality of cyclic heatings introduces a bridge unbalance equal to and in the same sense as the initial unbalance due to the departure of the measured temperature from the preselected control value, whereby the rebalance action developed by an increase in the effective value of bridge arm It must be double the rebalance action required to counteract the initial unbalance. This doubling of the rebalance action results in a final balance of the power input and power demand at the preselected control temperature value. v

By'transiorming the efiective control factors into equivalent thermal values, any desired degree of inertia and type of response is made available for use in the control circuit. An almost unlimited choice of heat input, heat transfer and heat storage characteristics permits design of the control circuit to obtain a response measurable in split seconds or in hours according to the particular process or apparatus to be controlled. Simplification of circuit arrangements and of mechanical constructions, improved stability and other advantages are thus characteristic of the invention. The effects of ambient temperature changes on the heat-responsive bridge arms l3, l4 cancel out and do not affect the ultimate accuracy of the control circuit.

The Fig. 1 control circuit is to be understood as illustrative of an application of the invention to the frequently encountered problem of the regulation of current to an electrical heater to maintain a substantially constant temperature.

The photocell type'of resistance in two arms of the bridge network necessitates relatively high resistances for the other bridge arms, but the invention is not limited to high resistance circuits or to the illustrated arrangement of the photocells since transformer couplings to obtain an impedance match with temperature-variant bridge arms are possible, or a resistor bulb and a fixed resistor could be substituted for the photocells. A control factor other than temperature may be employed to vary the relative illumination of the photocell or photocells of the control network, and amplifiers may be introduced between the tube and the control relay I9 if necessary or desirable. V

A-more elaborate embodiment of the invention which'includes means for manually adjusting the rate of rebalancing and/or resetting of the control system is illustrated in Fig. 2. The elements which are or may be substantially identical with elements of the Fig. l circuit are identified by the corresponding reference numerals but will not be individually described. The electronic tube 1 of the control network is of the pentode type with a control grid Ci upon which an initial negative bias is imposed through a temperature variant control network, and a second grid G which is returned to ground and the cathode K through a circuit having terminals 3:, 3 across which a control factor sets up a positive voltage to establish conduction through the tube under preselected conditions. The suppressor grid G is connected to cathode K and to the grounded side of the cathode heater H in conventional manner. The relay l9 in the plate circuit has contacts 20 for closing the circuit of a heater resistor 32 of a regulating device comprising a casing 33 having a diaphragm 3 1 dividing the casing into an air chamber 35 and a closed chamber 38 containing liquid 3'! in which the heating resistor?! is immersed. A rod 38 is secured to the diaphragm and is surrounded by a spring 39 which forces the diaphragm 34 downwardly upon the liquid in the closed chamber 36. The rod 38 is con nected to and actuates a lever 40 which is pivotally supported on the casing 33 and connected to a progressively adjustable regulating device and/or to the pointer of a measuring instrument, not shown.

The network for impressing a bias voltage upon the grid G takes the form of a complex Wheatstone bridge which includes a potentiometer 4! connected across the secondary winding 4 of the power transformer by leads 42, and a plurality of voltage dividers connected in parallel across the potentiometer 41; the individual voltage dividers comprising, in series, resistors I3 and it resistors I33 and His, and resistors 43 and 44. All of these resistors are preferably of the same magnitude when at the same temperature. Heating resistors 23 25f and 24s, 25s are in heat transfer relation to the serially arranged temperature-variant resistors I37, 14 and 13s, is, respectively, and these assemblies are so designed and constructed that, at heat saturation, the resistors 13 I4 I33 and Ms have the same temperature and same resistance value. The circuits of the heaters are connected across the secondary winding 5 of the power trans,- former, and controlled by contacts 2! of the relay [9, as previously described with respect to the Fig. 1 circuit;

The adjustable tap 41' of potentiometer 4| is connected by, a lead I! to the sliding tap of a resistor 45 which is connected across the second-" arr wind g. 5, o he power; r nsiorm n. are he cathod r d r u is co pleted thro gh he. res s an ne w rk by p n om e 45 connected between the mid-points or voltage dirid s 313. 1? a d .5 M a se D tiometer 41 connected between the tap. of poten tiometer 46 and the junction of resistors 43, 44, and lead Hi from the. tap of the second potentiometer to the grid G When the tap 4,1. of potentiometer 4| is at mid-point; and the tap of potentiometer 41 is adjusted to connect the grid lead I55 to the junction of the resistors 43,, 44, the grid bias is determined by the position of the tap on resistor 45 which is shunted aoross the cathode heater.

With the instantaneous polarity of; secondary windings 4, 5 and 6 as indicated on the, drawing for half-cycles during which the plate potential is positive, the temperature-variant resistors I -3f I33 are rebalancing elements and resistors 4f, Ms are resetting elements of the control net: work In each voltage divider, the rebalancing element l3 or |3s has. a lower heat inertia than its associated resetting element I41. or Ms; and

the heat inertias of. I31 and I4 are'less than the heat inertias of I38 and Ms, respectively. In other words, the voltage divider I37, [4 changes in. resistance more rapidly than the voltage divider l3s', |4s when heated or cooled for the same period, but the ultimate resistance, values of all sections of the voltage dividers are equal at heat saturation and after prolonged cooling.

The basic method of operation of the control network of Fig. 2 is the same as-that of the Fig. l circuit. A departure of the control factor from its preselected value, and in the sense which necessitates a clockwise. adjustment of lever 40 to rebalance the operating conditions develops a voltage across terminals :9, g which biases grid G? to establish conduction and energize relay |9.. This results in a current input to resistance 3.2 of the integrating regulator, and in current inputs to the heaters 24 25,1 and 24s, 25s to alter the absolute and. the relative values of temperature-variant resistors .of the control network The. efiective heat. inertias or time constants of the control elements may be varied by adjusting the tap of potentiometer .46, from the end adjacent the fast-acting voltage, divider [315, I41. to the. end adjacent the slow-acting voltage divider I35, I45, and the sensitivity of the control resulting from voltage changes developed at the selected time constant. may be varied by adjustment of the tap connece t'ion of. lead IE to. potentiometer 41, thereby deter--. mining the range of control. Adjustment of the tap 4| of potentiometer 4. I. permits a setting of the initial bias on grid G to. the sense and magnitude which establish a normal plate current approximately equal to one-half the m ximum plate current when the voltage across termi-E nals 1;, 11 corresponds to the preselected value of the control factor. F

The control factor may develop a direct current voltage across-the terminals 1:, y, or may develop an alternating current voltage of a fre%- quency up to high and ultra-high radio frequency signals in the case of, for example, a guided missile or rocket which is controlled by modulated radio frequency signals transmitted from an airplane or a home base. In place of asingle. tube with two control grids. two. tubes with a plate circuit in common may be. employed in the Fig. 2 control system.

The invention may be employed to. advanta e resistor 5| being connected to the terminal of I l9 to. derive; a: pulse modulation control from, and/or to add a resetting action to, conventional modulating control systems. In the Fig. 3 control system, a pair of resistive or inductive impedances Zl, Z2 are adjustable in oppositesense by changes in the, magnitude of the control factor as measured by any appropriate device. which may be, as indicated by, the legend, a'conventional modulating control. A tube and the associated power transformer for energizing the same are. substantially identical with corresponding elements of Fig. 1., and the component parts are identified by the same reference numerals but will not be described in detail. The control network includes a potentiometer 4| connected across the. secondary winding 4. of the power .t ansiorrner and a p u a f m rature varian vo di d -s s n d across the potentiometer. The. general arrangementv of the network issimilar to that. illustrated in Fig.

.2, but the voltage divider elements and associated heaters are identified by a new set or, reference numerals since. the functions of some of the voltage divider elements d-ifier from those of correspondingly arranged elements of the Fig. 2 assembly of temperature-variant voltage dividers. A pair of voltage-dividers comprising, in series, resistors 49f, 50a and resistors 49s, 59s are shunted across potentiometer 4|, the voltage divider 491, 50 having a. lower heat inertia than the voltage divider 49s., 5,118... A third voltage divider comprises resistors 51 and 52 in series, the

otentiom 4 w ic s at a. ne a ive po ential durin half-cycles when the plate potential of tube. is positive.

A heater 53, is in heat transfer relation to the s st r 2, which functions. as. the. resetting element of the control system) and the heater 53 is connected across. the cathodeheater H through t e contacts. 2! of the plate ci cu t relay 9. The resistor 5| is not provided with a heater and therefore is. of constant value except for variations with ambient temperature. Heaters 54f, 551

a and 54s, 55s are-in heat transfer relation to voltage divider resistors 49}, 50f and 498. 5.0.8, respectively. All of the voltage divider resistors arepreferably of the same value at any given temperature. A potentiometer 5,6 is connected between the. midpoints of the voltage dividers 4.9!, 5.02- nd .8 &8. and the tap. of the potentiometeris connected to the junction of resistors 5| 5.2 thrQ -l h a. second potentiometer 51, the. tap of the second potentiometer being connected to grid G oi tube by a lead 58..

Leads. 23, 2.3 connect thev grounded, positive ter-. minal of the heater H to, the junction of the control im edanees. Z|, Z2, and leads 59, .60 connect h o he t rminals o t e -d c x 2 to, h n en poten iome e M and. to th c h respectively. Heaters 54,), 54s for resistors 49], 49s. are, connected, in parallel with each other, betwe n lead- 9. tw n. impedanc Zl and a lead 6| to the negativ side. of the cathode heater H. Similarly, heaters 55], 58 for resistors 50], 50s are. connected, parallel with each other, between lead 60 "from impedance Z2 and the common lead 6| to heater H. Inspection of the described circuit arrangement shows that a Wheatstone bridge is formed by impedances Zl, Z2, heaters 541?, 54sin parallel, and heaters 55], 55s in parallel; and that this bridge is energized by he Qlte e drop ac oss. t e heat ri. e. leads 2.3. connect the. junction of the impedances Z11 Z! to he. nositiveistde of; the. heater H. and

11 lead 6| connects the opposite junction of the bridge to the negative side of the heater H. The other pair of opposite junctions of the bridge are connected, respectively, to cathode K by lead 60, and to grid G by lead 58 and the control network. This bridge network, when balanced, establishes the same potential on the grid C- and cathode K, but it is of course preferable to provide an initial grid bias of such negative value as to develop about one-half the maximum plate current in tube 1 when the control factor has its preselected value. The initial bias could be developed by a preliminary unbalance of the Wheatstone bridge, but it is preferable to develop the blocking bias by shifting the tap M of potentiometer 4| from midpoint towards the terminal which is positive when the plate potential is positive. This adjustment makes the oathode K positive with respect to the grid G which is connected to the midpoint of the voltage divider network by lead 58.

' With the system in operation, and assuming that the control factor has held its preselected value for a sufficient interval to permit the several heaters to cool to room temperature, the input Wheatstone bridge containing impedances Zl, Z2 is balanced, the complex Wheatstone bridge of the voltage divider system is balanced, and relay I9 is open at the plate current value determined by the negative bias resulting from the off-center adjustment of the tap M of potentiometer 4!. A change in the measured value of the control factor in that sense which calls for a power input results in action by the device 48 to decrease the value of impedance Zl and simultaneously to increase the value of impedance Z2. The Wheatstone bridge is thus unbalanced, and the immediate effects of the unbalance are to develop a less negative grid bias potential across the bridge, to increase the, current to heaters 54f, 54s and to decrease the current to heaters 55 55s. A preselected degree of bridge unbalance results in an increase in plate current to the value at which relay l9 pulls in to close the load circuit contacts 20 and also'the contacts 2! of the circuit of heater 53 associated with the re setting resistor 52. Voltage divider resistors 49] and 49s are heated more intensely by the increased current to their heaters 54 and 545, whereas resistors 50 and 5115 cool as the current to their heaters 55 f and 55sis reduced. The corresponding changes in the resistance values of these resistors reduce the positive potential of the junction of resistors 49 50 at a relatively rapid rate, and reduce the positive potential of the junction of resistors 49s, 50s at a slower rate. Heater 53 receives current, since the relay contacts 2| are closed, and the resultant heating of resistor 52 increases the positive potential at the junction of resistors I, 52, but at a still slower rate. The grid bias is shifted progressively in a negative sense by this differential heating of the resistive voltage dividers until the decreasing plate current is insufficient to hold in the relay IS. The relay contacts 20, 2! open to interrupt the power supply to the controlledcircuit, and to interrupt the heating current to heater 53 of the rebalancing resistor 52. This cycle is repeated when the measured value of the control factor drifts from its preselected value after the opening of the power supply at relay contacts 28, and the cyclic heating of the resetting esistor 52 operates after a time delay to reduce the positive potential at the junction of voltage divider resistors 5t, 52, thereby introducing an unbalancing action which supplements the unbalance of the control system as initiated by the adjustment of impedance Z! to a lower value than that of impedance Z2. The rebalancing resistors 49f, 50 and 49s, 58s must be heated for longer periods than would be required to compensate only for the unbalance of the impedances Z1, Z2, due to time-delayed unbalance introduced by resistor 52, and the eventual rebalance of the control system is effected at the average power input which restores the control factor to its preselected value at which the control device 38 adjusts impedances Z i, Z2 to equality.

The method of operation of the temperaturesensitive resistance network of Fig. 3 is generally similar to that of the resistive network of Fig. 2, but differs therefrom in that the rebalancing resistors are continuously heated and only the resetting resistor 52 is heated by current pulses to the heater 53. In the Fig. 3 circuit, the adjustment of the taps of potentiometers 56, 51 regulates'the time rate and degree of the resetting action of resistor 52.

The electronic tubes of the circuits so far described operate as amplifiers, but the current pulse method is also applicable to control systems in which the tube functions as an oscillator. The control system illustrated in Fig. 9 of Patent No. 2,434,941 is of the oscillator type, and a further oscillator system is illustrated in Fig. 4. The particular apparatus of Fig. 4 afiords a wired radio control of an oil burner of a home heating plant, but there are other applications in which the control factor is not temperature and in which the relay is directly included in the control system and not coupled thereto by a carrier wave link circuit.

The illustrated oscillator system includes a pentode 52 energized from a conventional power line L by a transformer having a primary winding 3, a low voltage secondary winding 5' across which the heater H is connected, and a high voltage secondary 6 which is connected between ground and. the screen grid G through the plate coil 63. The grid coil 64 is connected between grid G and the tap of of a potentiometer 45' shunted across the heater winding 5. Anode A is connected to the primary winding 3 by a condenser 65, and is connected to ground through a radio frequency choke 66, the winding of a relay fil'having contacts 68, and a blocking conjdenser 69.

The grid and plate-coils are coupled by a link circuit including coils 70, H of a few turns each inductively coupled to grid coil 64 and plate coil .53, respectively. The lead 72 which connects coils Hi, H is grounded on an electrostatic shield 13 which extends between the coupling coils and the oscillator coils, and the electrostatic shield is grounded through lead 14. The link circuit includes a coil 15 in series with the coupling coils iii, H; and a coil 16 shunted across the coupling coil ID. The primary temperature-variant control circuit is, in series, a coil 11 coupled to coil 15 and an impedance 13 which is adjusted as to 'eifective value by a thermostatic element 19. The temperature responsive circuit is grounded by a connection to the grounded lead 14.

' The rebalancing and resetting circuits of the control system include coils 8|, 82 coupled to coils l5 and 16 respectively, and grounded through temperature-variant resistors 83, 84 respectively. Heaters 85, 86 for resistors 83, 84 respectively are in series in a circuit across the heater 3; the seriescircuit comprising lead 81 from the negalive side of cathode heater H to the blade ele-' ment of the relay contacts 68, the stationary contact element, a lead 88 to the heaters, and the ground connections to the positive potential side of the heater H.

The power transformer which energizes the control system may be permanently wired to the line L but preferably the leads to the transformer terminate in a conventional plug connector 89 which may be inserted in any of the sockets 99 of the house circuits, thereby permitting a transfer of the control apparatus from one room to another. This is of considerable advantage over the present method of controlling the heating furnace according to the temperature at a preselected and fixed point within one room of the house. The temperature within that room can be held at a substantially constant level, but the temperatures in other rooms will'vary considerably with changing wind and weather conditions, and also with local heating from fireplaces and electric heaters. The plugein control apparatus permits a location of the control point of the system in the particular room which affords a temperature distribution throughout the house which best satisfies the operating conditions at the time.

'The controlled element of the Fig. 4 circuit is a burner 9| which is connected across the line L by contacts 92 of a relay 93 when the latter is energized by a radio frequency amplifier and detector 94. The signal input to the amplifier detector 94 is from line L through condensers 95, and the power supply to the amplifier-detector is from line -L through a conventional plug-in connection 96. Choke coils 91 are preferably included in the line L betweenthe burner and the radio frequency connections to the amplifierdetector unit 94, and also between the line L and the power source S.

The tube 62 is so biased by adjustment of the tap of potentiometer 45' that oscillation accompanied by an increase in plate current occurs at a predetermined degree of coupling of coils 63 and 64 through the link circuit. The coil 16 is in parallel with the link circuit, and an increase in its effective impedance will increase the degree of coupling and the tendency toward oscillation, whereas coil is in series in the link circuit and an increase in its effective impedance will decrease the degree of coupling and the tendency towards oscillation. The input or primary control impedance 18- is progressively adjusted to I lower values by the thermostatic element 19 as the room temperature rises, and the load imposed upon coil I6 by the primary control circuit, comprising impedance 18 in series with coil 11, therefore increases as the room temperature rises.

The resetting coil 82 is also coupled to coil 16, and the load imposed on coil 1'6 by coil 82 decreases as its serially connected resistor 84 is heated and increases in value. The effective impedance of the link-shunting coil 16 is therefore increased, with a corresponding decrease in the degree of link circuit coupling and tendency towards oscillation, when the resetting resistor 84 is heated by current through the heater 86. On the contrary, the heating of resistor 85 decreases the load imposed on coil 15 by rebalancing coil BI and increases the degree of link circuit coupling and the tendency towards oscillation since the coil 15 is in series with the link circuit.

The tube 62 is normally biased to develop about one-half the maximum plate current when the 'room temperature is at the preselected value, and

the rebalancingand resetting resistors 83, 84 respectively are at the same temperature and have identical resistance values. The relay 93 is de-ene'rgized and the power switch 92 of the burner is, open since there is no carrier wave input to the amplifier-detector 94.

A drop in the room temperature results in an increase in the value of impedance 1,8, thereby increasing the link circuit coupling, and the tube 62 oscillates when the preselected degree of coupling is reached. Carrier wave energy is transmitted to the amplifier-detector unit 94, and relay 93 is energized to close the contacts 92,

thereby starting operation of the burner 91 The plate, circuit relays! also pulls in to close switch 68 and connect the heaters 85, 86 across the secondary winding 5 of the power transformer. The heat transfer to resistor 83 increases its effective value, thereby decreasing the load imposedon coil 15 by the rebalancing coil 8|, and the degree of coupling is thereby reduced until oscillation is blocked by the fixed bias from potentiometer 45'. The resetting resistor 84 is simultaneously increased in effective value by heat transfer from heater 86, but the thermal when the degree of coupling of the oscillator coils 63, 54 by the link circuit rises to its preselected value and, after a few cycles, a balance is reached between the average heat input and the heat demand to maintain the room temperature at the desired level.

It is to be noted that the plate circuit relay 6'! can be omitted by designing the oscillator to jump into and to drop out of a state of oscillation, and locating the heaters 85, 86 in the plate or screen grid circuits.

The entire power supply to the controlled device is subjected to an on-off control in the apparatus as illustrated in Figs. 1, 3 and 4, but it is to be understood that there may be a continuous supply of power to the controlled device at a level sufficient to meet the minimum power demand, and'that the control will then be imposed only upon a complementary power supply of a magnitude sufficient to increase the total power input to or above the anticipated maximum value.

The illustrated circuits indicate the wide latitude in the design and construction of apparatus operating in accordance with the invention, and it is to be understood that other circuit arrangements which may occur to those familiar with -the art fall within the spirit and scope of the invention as set forth in the following claims.

I claim:

1. An electronic measuring or control apparatus for equipment to which a medium is to be supplied to maintain a control factor at a preselected value, said apparatus being of the type including an electronic tube having a control grid cooperating with a cathode and an anode, energizing means to energize said electronic tube, said energizing means including a first current source for developing a grid bias voltage and a 15 second current source for developing a cathodeheating current, an input circuit network for said tube including a bridge network havingone set of conjugate terminals connected across said first current source and a second set of conjugate terminals connected respectively to said control grid and to said second current source, means for imposing upon said input circuit network a control voltage varyi g with the value of said control factor, thereby to alter the current output of the tube, an output circuit for said tube having therein current-responsive means effecting a cyclic on-off control of the supply of said medium effecting the magnitude of said control factor, and means for converting said on-oif control to a proportioning control varying with the magnitude of the instantaneous departure of the value of said control factor from the preselected value; characterized by the fact that said last-mentioned means comprises means developing a secondary control voltage in said bridge network to impose on said control grid a bias voltage varying in magnitude with the integrated time durations of those portions of the on-off cycles during which the medium is supplied to the equipment.

2. An electronic measuring or control appa- V ratus as recited in claim 1, wherein both said control voltage and said secondary control voltage are impressed upon said control grid.

3. 'An electronic measuring or control apparatus as recited in claim 1 wherein said electronic tube includes a second grid, and said input circuit includes a circuit connected to said second grid and included in said means for imposing said factor-varying control voltage on said input network. i

4. An electronic measuring or control apparatus as recited in claim 1, wherein said input circuit network is non-resonant and said electronic tube operates as an amplifier.

5. An electronic measuring or control app tus as recited in claim 1, wherein said means for developing a secondary control voltage includes a temperature-variant impedance constituting a side arm of said bridge network, a heater resistor for said impedance, and a circuit for said heater resistor including a current source that is cyclically closed and opened by said current-responsive means in synchronism with the cyclic on-ofi' control of the supply of said medium.

6. An electronic measuring or control apparatus as recited in claim 5, wherein said current source of the heater resistor is said second current source.

'7. An electronic measuring or control apparatus as recited in claim 5, wherein the said currentresponsive means is a relay having one set of contacts controlling the supply of said medium, and a second set of contacts in the circuit of said heating resistor.

8. In an electronic control apparatus for regulating the supply of a medium to equipment for maintaining a control factor at substantially a preselected value; an electronic tube having a grid cooperating with a cathode and an anode, energizing means to energize said tube, said energizing means including a first current source for developing a bias voltage and a second current source for developing a cathode-heating current, an input circuit network between said grid and cathode, said network including impedance forming a Wheatstone bridge, an output circuit connected between the anode and cathode, said output circuit having therein a relay with contacts imposing an on-oif control upon the supply of the medium to said equipment, means including instantaneous departure of said control factor from preselected value and in the sense resulting from an inadequate supply of the medium to increase the tube output current to above the pullin current value of said relay, thereby to establish a further supply of the medium, and timedelayed means energized by a pull-in of said relay to alter the balance condition of said Wheatstone bridge to reduce the tube output current below the drop-out current value of the relay, thereby to interrupt the supply of said medium to the equipment. 4

9. In an electronic control apparatus, the invention as recited in claim 8, wherein the arms of said Wheatstone bridge include two phototubes and two temperature-variant impedances, said means to increase the tube output current includes means responsive to the changing magnitude of the control factor for regulating the relative illumination of said phototubes, and said time-delayed means includes heater resistors in heat-transfer relation to said temperature-variant impedances,- said heater resistors being in series circuit with said second source of current and contacts of said relay; I

10. In an electronic control apparatus, the invention as recited in claim 9, wherein the heating of one temperature-variant impedance'varies the bridge balance in that'sense whichtends to decrease the tube outputcurrent while theheatin of the ,second-temperature-variant impedance tends to increase the tube output current, the heat inertia of the first temperature-variant imped: ance and its associated heater resistor being less than that of the second temperature-variant impedance and its associated resistor; whereby the first temperature-variant impedance functions to rebalance the input circuit network to ole-energize the relay at a value of the control factor other than said preselected control factor value, and the second temperature-variant impedance functions'to reset the input circuit network at balance at the preselected value of the control factor.

11'. In an electronic control apparatus, the invention as recited in claim 8, wherein said tube includes a second grid, and said means for increasing the tube output current inoludes a gridbiasing circuit between the cathode and-said second grid. m 7

12. In an electronic control apparatus, the invention as recited in claim 8, wherein a plurality of the arms of said Wheatstone bridge are temperature-variant impedances; and said time-delayed means includes heater resistors in heattransfer relation to said temperature-variant impedances, and heating circuits for said resistors controlled by said relay; one temperature-variant impedance being a rebalancing impedance operating on heating thereof to reduce the tube output current and a second temperature-variant impedance beinga resetting impedance operating on heating thereof to increase the tube output current; said resetting impedance having a greater heat inertia than said rebalancing impedance; in combination with means energized by said relay when pulled-in for an interval in excess of that required for said resetting impedance to reach its saturation temperature to open the heating circuit of the heater resistor in heat transfer relation to said resetting impedance. I

13. In an electronic control apparatus, the invention as recited in claim 12, wherein said relayenergized means is a thermal switch having a heating circuit controlled by said relay.

14. In an electronic control apparatus, the invention as recited in claim 8,. wherein said Wheatstone bridge is complex and comprises a potentiometer across which said first current source is connected as an element of said means for imposing a preliminary bias on the tube grid, the terminals of said potentiometer constituting an opposite pair of junctions of said Wheatstone bridge, a plurality of resistive voltage dividers to said temperature-variant resistances, and

heating circuits for said heater resistors including said second current source and contacts of said relay; the heat inertias of the temperaturevariant resistances of two voltage dividers being different.

15. In an electronic control apparatus, the invention as recited in claim 14, wherein said re sistance means includes a potentiometer connected between the junctions of the sections of said voltage dividers of different heat inertias, and a resistor connected between the junction of said first potentiometer, said second potentiom eter having a tap included in said means connecting the junction of the section of said third voltage divider to the tube grid whereby adjustment. of the tap of said second potentiometer .renders the grid bias independent of the relative resistance values of the respective sections of said voltage dividers having temperature-variant resistances.

I 17., In an electronic control apparatus, the in vention as recited in claim v14, wherein said in- I put circuit network includes a second Wheat-v stone bridge having a conjugate pair of terminals connected to the second pair of opposite junctions of said first Wheatstone bridge, the other conjugate pair of terminals of said second Wheatstone bridge being connected across said first current source; said second Wheatstone the sections of said third voltage divider and the tap of said last-mentioned potentiometer whereby the time-rate of changein the balance condition of the Wheatstone bridge varies with the ad- 7 justment of the tap of said last-mentioned .potentiometer.

of said voltage dividers of different heat in ertias, a second potentiometer having ends connected between the junction of the sections of said third voltage divider and the tap of said first potentiometer whereby the time-rate of change in the balance condition of the Wheatstone bridge varies with the adjustment of the tap of bridge having two bridge arms comprising impedances adjustable in opposite sense by variations in the instantaneous value of the control factor,

and two arms of said second Wheatstone bridge comprising, in parallel, the heater resistors for said temperature-variant resistances constituting A sections of two voltagedividers having different heat inertias. I 18. In an electronic control apparatus, the invention as recited in claim 14 wherein the second section of said third voltagedivider is a temperature-variant resistance, and said timedelay means includes a heating circuit for said last-mentioned temperature-variant, resistance said relay.

and controlled by e GEORGE A. F. MACI-ILET,

References Cited in the file of this patent UNITED STATES PATENTS j Number Name Date 1,497,194 Norden June 10, 1924 1,822,075 Aronoff Sept. 8, 1931 2,154,375 Chambers Apr. 11, 1939 2,235,169 Roberts Mar. 18, 1941 2,263,298 Haimbaugh Nov. 18', 1941 2,276,506 Moore Mar. 17, 1942 2,278,633 Bagnall 'Apr. 7, 1942 2,290,091 Brown July 14, 1942 2,299,330 Macnabb Oct. 20, 1942 2,300,537 Davis Nov. 3, 1942 2,325,232 Davis July 27, 1943 2,325,308 Davis July 27, 1943 2,423,534 Upton July 8, 1947 2,477,835 Smith Aug. 2,1949

Davis Feb. 7, 1950 

